Bar grip

Deliberate shaping of moulded tyre tread design, rather than merely concentrating on a rubber surface that didn't fail rapidly, began at the start of the 1920s. There was little theoretical background to this though until the mid-1920s. The 1922 London Motor Show displayed a range of tread patterns that were reported on by The Autocar.[2] Some of these, from Miller and the French firm of Bergougnan, display what would now be recognised as the classic bar grip pattern of sharp-edged transverse bars with a wide central rib. However these were supplied for narrow high-pressure tyres, for use on roads and at relatively high speeds. Their design was based on simple notions of transverse bars giving fore-and-aft traction, without any developed theory of tread design or experimental measurements of their performance. In practice, such square-edged tread blocks wore badly and unevenly,[3] the initially flat blocks tending to wear with a sloping top surface. This wear pattern was one reason for the practicing of rotating tyres between wheels, so that they spent equal periods wearing in each direction.

Later, and better, patterns were the Dunlop 'Traction' tread pattern of 1922 and the 'Triple stud' pattern of 1927. These followed the same principle of a solid central rib with square-edged blocks, but were developed to improve tread wear. The central rib became a thin and almost vestigial rib between square blocks. The side blocks were now isolated from the centre, although linked by another narrow rib, and their leading and trailing edges formed a trapezium rather than a square.[3] This triple square stud pattern was a successful pattern for narrow tyres, including motorcycles, and remained popular from several manufacturers into the 1950s.

Alvis Saracen on late bar grips, with helical skew

The first deliberate use bar grip tyres as off-road tyres, were developed pre-war as truck tyres particularly for army use. They used crosswise tread bars for grip. arranged in a helical pattern.[4] A helical pattern was used to avoid the uneven rolling radius and vibration of a tread with simple crosswise bars and large gaps between. This helical pattern also gave rise to a sideways force, particularly on tarmac. As all tyres had the same 'hand', this force increased across the two sides of the vehicle. To counteract this force, the central stop-rib was added.[4] The stop-rib was only partly successful though; snowy conditions led the Swiss Post Office to use left- and right-handed tyres to balance the force across the vehicle. Later developments would use balanced arrangements of ribs in a chevron or herringbone layout.[4][lower-roman 1] The more common arrangement for the later low-speed military bar grip was to use straight bars, but relying on the centre rib to avoid their vibration problems on tarmac.

All bar grips are crossplies, having been developed when this was the only practical means of manufacturing tyres. The simple pattern of bar grips also made them easy to manufacture and hard-wearing, even if made from poor quality or ersatz rubber, bulked up with extreme quantities of non-rubber fillers.